1. Field of the Invention
The present invention relates to techniques for fabricating a semiconductor chip. More specifically, the present invention relates to a method and apparatus for fabricating a semiconductor chip using patterns that have varying levels of precision for different regions of a reticle that is used to lithographically expose the semiconductor chip.
2. Related Art
Semiconductor foundries use lithographic techniques to create modern semiconductor chips. During the semiconductor fabrication process, light passes through a photomask within a reticle and exposes a photoresist layer previously spun onto a wafer. This light defines patterns on the surface of the wafer that can be filled with metals or that can be implanted with dopants, thereby creating transistors and wires. Presently, ultraviolet light, with a wavelength of 193 nm, is used to expose the photoresist layer.
The minimum feature size supported by a given lithographic technology does not need to correspond to the wavelength of light which is used. For example, present lithographic technologies support minimum feature sizes under 50 nm, yet can be accurately imaged with light that has a much larger wavelength (for example, 193 nm). This seeming contradiction is made possible by several resolution-enhancing techniques, such as phase-shifting masks, off-axis illumination, and optical proximity correction. Unfortunately, as lithographic technology scales to finer and finer feature sizes, the ability to focus coarse wavelengths of light through very finely patterned reticles becomes more difficult.
As lithographic technology approaches the diffraction limit of light, the maximum reticle (photomask) size decreases. Consequently, even with the advanced resolution-enhancing techniques available to foundries, the maximum region of lithographic “focus” continues to narrow. This reduces the individual die size available to VLSI designers.
One solution to this problem is to use 157 nm wavelength light, which helps resolution, and thus increases the maximum reticle size. Unfortunately, lithography systems which use 157 nm wavelength light require photomasks made of calcium fluoride, which exhibits polarization-dependent light refraction (or “birefringence”). This characteristic is causing significant problems. Consequently, lithographic systems which use 157 nm wavelength light are not expected to hit production for several years. Furthermore, 157 nm wavelength light only improves the situation marginally over 193 nm wavelength light.
Another solution is to use extreme UV (EUV) light, or to use a wavelength of light near 30 nm. This solution dramatically reduces the reticle focusing issues, but EUV is even further away from production than 157 nm wavelength light. Furthermore, EUV light has several problems including being easily diffracted and easily absorbed in air.
Hence, what is needed is a method and an apparatus for fabricating semiconductor chips without the problems described above.